The present invention relates in general to servovalves and in particular to fail-fixed servovalves which use positive fluid feedback.
It is known to use servovalves at the interface between an electrical control system and different types of mechanical or hydraulic actuating devices. For example, in a gas turbine engine fuel control system the servovalve may control the movement of a servopiston in response to an electrical control signal.
In certain types of control systems, it is desirable to use a fail-fixed servovalve for controlling the movement of the servopiston. The expression fail-fixed servovalve, as used herein, designates a servovalve which has no mechanical output in the event the electrical control signal is either lost or exceeds a maximum rated control signal value, i.e. the servopiston is locked in position when these situations occur. An example of a servovalve which is fail-fixed when the control signal is lost is described in U.S. Pat. No. 4,276,809, assigned to the assignee of the present invention. The fail-fixed servovalve described in the above-mentioned patent utilizes a shuttle piston which blocks the flow of fluid to a servopiston chamber when the control signal falls below a predetermined value. Such a servovalve is, however, fail-fixed only for control signals which do not exceed a predetermined value.